The thermal decomposition of NO2Cl in Ar and in O2 as well as that of pure NO2Cl has been studied from 600° to 1000°K at concentrations of ∼10—3 to ∼10—2 moles/liter using a shock-wave technique. In general the results are compatible with earlier, low-temperature measurements. The activation energy for pure NO2Cl decomposition is about 2.5 kcal/mole less than that for NO2Cl+Ar or NO2Cl+O2. This is compared to the drop in activation energy calculated from the differences in energy transfer efficiency assuming deactivation on every collision.